The steroid hormones, estradiol and progesterone, are released by the ovaries and act in brain to influence a variety of physiological and behavioral events, including energy homeostasis and female reproduction. Disruption of ovarian hormone action in the ventromedial nucleus of the hypothalamus induces metabolic syndrome and abolishes female reproductive behavior in rodents. These hormones mediate many of their effects by binding to their respective receptors. While it has been accepted that steroid receptors can be activated in a classical ligand-dependent fashion, there is mounting evidence that receptors can be activated in the absence of hormone in a ligand- independent manner. For example, progestin receptors (PR) in brain can be activated by dopamine in the absence of progesterone to elicit profound effects on female reproductive behavior. However, very little is known about how steroid receptors activate genes in the brain, in a ligand-dependent or - independent manner, to ultimately cause changes in physiology and behavior. Nuclear receptor coactivators dramatically enhance the transcriptional activity of steroid receptors. While in vitro studies have revealed much about the molecular mechanisms of coactivator action, we are only beginning to understand coactivator function in brain to activate steroid-responsive genes and to regulate hormone-dependent behavior and physiology. This proposal investigates the function of two important coactivators, steroid receptor coactivator-1 (SRC-1) and SRC-2, in ligand-dependent and -independent PR action in mouse brain and behavior. Aim 1 will investigate the function of SRC-1 and SRC-2 in estrogen receptor (ER)-mediated induction of the PR isoforms in mouse brain. In support, we have identified estradiol-induced PR cells that coexpress both SRC-1 and SRC-2 in brain regions known to regulate energy homeostasis and female reproductive behavior. PR isoform specific knock-out mice will be used to test the hypothesis that SRC-1 and SRC-2 are important in ER-mediated induction of the two PR isoforms. Aim 2 will test the hypotheses that SRC-1 and SRC-2 are important in progesterone-dependent and -independent (dopamine-activated) PR facilitation of receptivity in mice. Aim 3 will test the hypotheses that coactivators from mouse brain physically interact with mouse PR when activated by progesterone or by dopamine in the absence of progesterone. In addition, mass spectrometry will be done to identify other coactivators, and potentially novel proteins, that interact with PR when activated by progesterone or dopamine. Collectively, these studies offer novel concepts and propose innovative combinations of approaches that will enhance our understanding of the molecular mechanisms underlying steroid action in brain and behavior and increase our understanding of the role of these hormones in human disorders, including metabolic syndrome. PUBLIC HEALTH RELEVANCE: The steroid hormones, estradiol and progesterone, act via their respective receptors in brain to influence a variety of physiological and behavioral events, including energy homeostasis and reproduction. Combining a behavioral- and proteomics-based approach, we seek to better understand basic mechanisms of steroid receptor action in brain and behavior. Investigating these mechanisms of estrogen and progestin action in brain will enhance our understanding of the role of these hormones in human disorders, including metabolic syndrome.